This application claims the priority of German patent documents EP 98103365.7 and DE 197 36 138.2, filed Feb. 26, 1998 and Aug. 20, 1997, respectively, the disclosures of which are expressly incorporated by reference herein.
The invention relates to a process and apparatus for determining the condition of a road surface in which backscattered light is divided into spectral components, and the wavelength-dependent course of the backscatter intensity is analyzed.
The process is specifically used for qualitatively, and preferably quantitatively, detecting the presence of water or ice on the road surface and can be implemented via a system mounted in a vehicle.
A process of essentially this type, described in German Published Patent Application DE 41 33 359 A1 makes it possible (in a no-contact manner) to detect the thickness of a water layer on a road. For this purpose, the light beamed onto the road surface contains at least two wavelengths from the short infrared range, which are subjected to a water absorption of different intensities. For example, for a sensitive measuring range, a measuring wavelength at 1,450 nm and a comparative wavelength at 1,190 nm with a comparatively lower absorption are selected. For an insensitive measuring range, a measuring wavelength of 1,190 and a comparatively even lower absorbed comparative wavelength of 1,080 mm are selected. Assuming an exponential law of absorption and a "gray" road surface (that is, one which absorbs all wavelengths at the same ratio), the thickness of a layer of water present on the road surface is thus determined.
The assumption of a gray radiator for the road surface is critical. In fact measurements on various road coverings have demonstrated that it is valid only to a limited extent. Some road coverings, for example, exhibit a noticeable rise of their spectral reflection capacity with an increasing wavelength. A demand therefore exists for a process which is largely independent of the spectral reflection capacity of the road surface. In addition, a process is desirable which, as required, can be designed such that it is also suitable for recognizing ice or snow coverings on the road surface.
German Patent Document DE 41 41 446 C1 discloses a process for measuring the thickness of a layer of water, snow or ice on a road surface based on run-time measurements.
German Published Patent Application DE 30 23 444 A1 discloses a device for determining road conditions, in which infrared radiation having a wavelength at which the reflection capacity of snow is lower than that of dry road surface, is beamed onto the road surface. The backscattered light is then compared with reference signal levels which correspond to previously classified conditions of the road surface.
In the process for monitoring the condition of a road described in German Patent Document DE 38 41 333 C2, electromagnetic radiation is beamed onto a tread surface of a vehicle wheel that is in contact with the road, and backscattered radiation is analyzed.
German Published Patent Application DE 35 45 366 A1 discloses a measuring device for visually determining the thickness of a film of water situated, for example, on a metallic surface, in which light is beamed onto the film of water. The light contains a measuring beam of a wavelength that is absorbed by the film of water, and a reference beam of a wavelength which is subjected to no water absorption. The thickness of the film of water is then determined from the ratio of the measuring signal level representing the backscattered measuring beam fraction to the reference signal level representing the backscattered reference beam fraction, multiplied with a temperature correction coefficient. The measuring signal level and the reference signal level are optionally averaged over several measuring operations.
German Published Patent Application DE 40 40 842 A1 discloses an infrared microwave sensor system for recognizing the road condition (particularly, for recognizing whether the road is dry, wet or icy). This system contains networks consisting of one or several comparators, or one or several gates or flip-flops for processing the signals of an infrared or microwave receiver. Here the backscattered intensity of the electromagnetic radiation is analyzed for two different wavelengths or narrow wavelength bands and the quotient of these two intensities or of detector signal voltages derived therefrom is formed. According to the value of the determined quotient, a conclusion is drawn with respect to whether the road is dry, wet or icy.
Non-prior art German Patent Application DE 197 36 138 discloses a process for determining reliably and precisely the condition of a road surface (particularly with respect to the presence of water, ice and/or snow), at relatively low expenditures, and independently of the road covering. In this process, light beamed onto the road surface contains at least a first light ratio from a first wavelength range without any significant absorption by water or ice (including snow), and a second light ratio from a second wavelength range with a significant absorption by water/ice or snow. At least the first light fraction contains light of several different wavelengths. (The term "without significant absorption" means (in this case) that although there may be a certain low absorption, it is clearly lower than in the case of the wavelength range with a significant absorption.) The light backscattered by the road surface is detected and spectrally analyzed. For this purpose, an approximation reference curve is determined based on the spectral data obtained for a first backscattered light fraction pertaining to the several different wavelengths of the first light fraction. This can take place, for example, by determining the approximation reference curve as a parameterized compensating curve over the individual measuring points of these spectral data by using a customary compensating calculation. The approximation reference curve extends along the whole spectral course of the backscattered light, thus also along the wavelength range of the second light fraction with a significant absorption by water/ice. Starting from the measuring points in the wavelength range of the first light fraction (without significant water/ice absorption), the approximation reference curve is interpolated or extrapolated into the wavelength range of the second light fraction.
In this manner, the approximation reference curve forms a reference curve in the wavelength range of the second light fraction. This reference curve reflects at least, approximately, the spectral course of the backscattered light in this wavelength range in the case of a dry road surface. In the case of a wet or icy road, the spectral data of the backscattered light in the second wavelength range deviates noticeably from the approximation reference curve, because of corresponding absorption of the second light fraction by the water or ice. In a similar manner, the process also provides the determination of the difference between the spectral data of the second backscattered light fraction originating from the second light fraction and the corresponding data of the approximation reference curve for at least one wavelength of the second wavelength range. In order to become independent of the overall light intensity, the difference thus determined is preferably standardized to the overall light intensity, or to the fractions in the non-absorbing wavelength ranges, and provides a reliable qualitative as well as quantitative measurement for the presence of water or ice on the road surface.
In the described process, the first and the second light fractions, which are relevant to the analysis of the backscattered light, are preferably in the wavelength range of between approximately 800 nm and approximately 1,100 nm. This provides the advantage that the wavelength range (without a significant water/ice absorption) is composed of two partial ranges below and above the second wavelength range. As a result, measuring points for determining the approximation reference curve for the spectral course of the backscattered light (in the case of a dry road) exist on both sides of the second wavelength range. Subsequently, this spectral course can be precisely interpolated for the wavelength range in the case of significant water/ice absorption. Another advantage gained in the case of the selected wavelength range is that the penetration depth of the light is sufficiently large. This provides the ability to completely penetrate the water or ice layers typically existing on traffic routes. Furthermore, reasonably priced silicon photodetectors can be used as backscattered-light detectors and no special infrared detectors are required.
A similar process which is disclosed in International Patent Document WO96/26430. Furthermore, German Patent Application DE 197 36 138 discloses a further development of the process, in which the second light fraction also contains light of several different wavelengths. The difference between the spectral data of the backscattered light originating from the second light fraction on the other hand, and the corresponding data of the approximation reference curve is determined not only at a single point but at a corresponding number of points as a difference curve.
From the course of the difference curve, the condition of the road surface can be qualitatively and quantitatively determined in a relatively precise manner. For this purpose, among other things, the integral of the difference curve is determined via the second wave range. The resulting (preferably standardized) integral, forms a very precise measurement of the thickness of a film of water which may have formed on the road surface.
In a further development as set forth in WO 96/26530, the center of gravity of the difference curve is determined. It was found that, as the result of this knowledge of the center of gravity, a decision can be made as to whether a covering situated on the road surface is a water layer or an ice/snow layer. In the case of snow, the difference curve is similar to that for ice but has a lower amplitude so that an additional discrimination can be made between ice and snow.
Since the position of the center of gravity of the difference curve shifts toward larger wavelengths as the temperature of the water layer or ice layer decreases the temperature of the water layer or ice layer can be determined from the magnitude of the shift. The knowledge of the temperature of the water layer or ice layer (in addition to furnishing knowledge of the layer thickness), supplies valuable information for estimating the presence of dangerous slippery ice. If water is present on the road, and if the water temperature approaches the freezing point, the driver can be warned so that he has enough time to anticipate freezing conditions.
The prior processes for determining the condition of a road surface have the disadvantage that they require, as a prerequisite, the existence of ranges above or below the absorption wavelength of water/ice, in which no absorption takes place, and which can therefore be used for the reference formation in order to compensate the influences of the road surface. This prerequisite cannot always be met. This is particularly true in the case of larger wavelengths of above 1,100 nm. As a result, areas without any significant absorption by water/ice can no longer be isolated, and a reference curve cannot be formed.
The object of the present invention is to provide a process and apparatus with which the condition of a road surface can be determined.
Another object of the invention is to provide a process and apparatus for determining the presence of water, ice and/or snow on a road surface, reliably and precisely, with relatively low energy expenditures and independently of the road covering.
These and other objects and advantages are achieved by the process and apparatus according to the invention, in which backscattered light is divided into spectral components, and a wavelength-dependent plot of the backscatter intensity is analyzed. For this purpose, the wavelength-dependent backscattered-light spectrum of a dry road surface (road scatter coefficient) is arranged via a mathematic function having free parameters. This function is multiplied with an exponential absorption term which describes the light transmission of a water layer of a certain thickness in order to obtain a mathematical function for the wavelength-dependent course of backscatter intensity as it impinges on the receiver. When forming the absorption curves of water or ice (which are entered into the absorption term), the actual absorption curves of water or ice can advantageously be used as a basis for forming the absorption curves. By means of the actual measured values, the free parameters of the function can then be calculated in the sense of a best possible approximation. This among other things, results in a determination of the thickness of the water layer.
In the process according to the present invention, the backscatter intensity is formed by a mathematical function with free parameters. With the justified assumption that the contributions of the detectors are linearly independent of one another, each contribution furnishes an equation for an equation system which can be used for determining the model parameters. Here, the analysis will be simplified if .delta.-functions are used for the spectral sensitivity of the detectors (that is, if the spectral plot is analyzed in a punctiform manner, for example, point-by-point, interpolation, at different discrete wavelengths.
In another embodiment of the invention, the information concerning the backscattered light need not be present spectrally; rather, several detectors having different spectral sensitivities can be used. Their converted light intensity will be obtained as the integral of the wavelength via the product of the spectral sensitivity of the respective detector with the spectral backscatter intensity.
This analyzing process according to the invention has the additional advantage that it can also be used when the spectral information of the backscattered light is detected only indirectly. In particular, the process can be applied to a measuring arrangement that has a broad-band light source and broad-band detectors with different respective spectral sensitivities. In this case, the information concerning the plot of the curves when the road is dry and an absorption-caused downward shift of the spectral plot of the backscattered light are always present (that is, mixed) in the receiver signals.
In yet another advantageous embodiment of the process according to the invention, interference effects caused by ambient light can be mathematically eliminated by utilizing differentiating spectral characteristics between the ambient light and the sensor light. This is advantageous in comparison to other known processes, where a pulsed light source (chopper) must be used in order to eliminate the influence of ambient light.
In still another advantageous embodiment, the system according to the invention is a spectrometer which can be used not only for recognizing road conditions, but for all applications in which a spectrum must be measured. In contrast to known spectrometers, an absorbing element is used instead of a dispersive element. In this case, the absorbing element is applied directly to a CCD sensor line. In a specific version of the absorbing element, the spectral composition of the light can be determined from the measured intensities while using an inverse Fourier transform.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.